Place ground cherries in a food processor; pulse until coarsely chopped. Transfer to a large saucepan.
Stir sugar, lemon zest, and lemon juice into the cherries. Bring to a boil; cook, stirring occasionally, until ground cherries soften, about 15 minutes. Add lemon gelatin; stir until dissolved. Skim any foam off the top of the jam with a slotted spoon.
Pour jam into two 8-ounce freezer-safe containers. Let stand until thickened and set, about 30 minutes. Transfer to the freezer.